1. Field of the Invention
The present invention relates to a piezoelectric transformer preferably for use in an inverter for a liquid crystal display back light, in an inverter for lighting a fluorescent lamp, or in a high-voltage power supply circuit for copying machines or the like.
2. Description of the Related Art
A conventional piezoelectric transformer is constructed as shown in FIG. 8. FIG. 8 is a diagram showing an external appearance, input/output configurations and a vibrating form of the piezoelectric transformer vibrating in the secondary vibration mode of longitudinal vibration ordinarily called .lambda. mode. The resonance wavelength of the longitudinal vibration used is hereinafter referred to as .lambda..
This piezoelectric transformer includes a flat rectangular piezoelectric plate 1 having a length of .lambda. and formed of a piezoelectric ceramic. Input electrodes 2a and 2b are disposed on opposite positions on two major surfaces of one of two halves of the piezoelectric plate 1 bisected along the width direction of the piezoelectric plate 1. An output electrode 3 is formed on one end surface of the other half of the piezoelectric plate 1, the electrode 3 extending along the width of the plate 1. The half of the piezoelectric plate 1 which forms an input section is polarized in the direction of thickness of the plate 1 , as indicated by arrow P1, while the other half of the piezoelectric plate 1 forming an output section is polarized in the longitudinal direction, as indicated by arrow P2.
This piezoelectric transformer vibrates so that, as shown in the lower section of FIG. 8, a nodal point at which the vibration displacement is zero is located at a distance corresponding to 1/4 of the length of the piezoelectric plate 1 (.lambda./4) from each of the lengthwise opposite ends of the piezoelectric plate 1 while the vibration displacement is maximized at the opposite ends.
This piezoelectric transformer is fixedly supported at positions corresponding to the nodal points, and input wirings 4a and 4b are respectively connected to the input electrodes 2a and 2b at positions corresponding to the nodal point while an output wiring 5 is connected to the output electrode 3. The input wiring 4b functions as an input-output common terminal (grounding electrode). An input voltage is applied between the input electrodes 2a and 2b through the input wirings 4a and 4b, and an output voltage generated by the piezoelectric effect and the reverse piezoelectric effect is output through the output electrode 3 and the output wiring 5. Lead wires or the like are used as wirings 4a, 4b, and 5 for connection to external circuits by being connected to the input electrodes 2a and 2b and the output electrode 3 by soldering.
The connection of the input wirings 4a and 4b may be made at any position as long as the connection position corresponds to the nodal point. It may be such that portions of the input electrodes 2a and 2b are formed on the side surfaces perpendicular to the widthwise direction as connection electrodes electrically connected to the input electrodes 2a and 2b, and the input wirings 4a and 4b are connected to the connection electrodes at positions on these side surfaces corresponding to the nodal point.
If the fundamental (primary) vibration mode commonly referred to as the .lambda./2 mode (not shown) is used, a nodal point occurs at the position corresponding to 1/2 of the length of the piezoelectric plate 1 (.lambda./2) in the lengthwise direction and the vibration displacement is maximized at the opposite ends of the piezoelectric plate 1. In the tertiary (3.lambda./2) vibration modes or other higher-order vibration modes, a nodal point exists at the center of each .lambda./2 section and the vibration displacement is maximized at the opposite ends and at the positions corresponding to integer multiples of .lambda./2 from the opposite ends of the piezoelectric plate.
In the above-described piezoelectric transformer, the piezoelectric plate is ordinarily supported at positions corresponding to locations of nodal points at which the displacement caused by vibration is minimum. Electrical connection between the input electrodes and an external circuit is also made at a position corresponding to a location of a nodal point in order to avoid any impediment to vibration and to prevent disconnection of lead wires or the like due to vibration. Also, input wiring connections are made at positions on the side surfaces corresponding to a location of a nodal point for the purpose of reducing the overall thickness of the piezoelectric transformer.
At a nodal point, however, the vibration stress is maximum while the vibration displacement is minimum. Therefore, even though the piezoelectric plate is supported on a supporting member or electrical connection by soldering at a nodal point in the abovedescribed conventional piezoelectric transformer, the state of bonding or joining to the supporting member or the soldered state of lead wires or the like is liable to deteriorate, so that the reliability of supporting the piezoelectric plate and of the various electrical connections to the piezoelectric plate is low.
For stable support and practical mounting, support or electrical connection is ordinarily made in the vicinity of widthwise opposite ends or on side surface portions corresponding to a location of a nodal point. In such a case, however, vibration in the widthwise direction is limited so that a deterioration in performance, i.e., a reduction in step-up ratio or the like, results, or the reliability of electrical connection is reduced by a vibration stress in the lead wires or the like due to the widthwise vibration displacement.